1. Field of the Invention
This invention relates to a method for improving the environmental quality of water which contains undesirable inorganic sub-six-sulfur-containing impurities. More particularly, it relates to a method for improving a water stream by converting the sub-six sulfur of a sulfur-containing impurity present in the stream to sulfate. By "sub-six sulfur" as used herein is meant sulfur having a valence (oxidation state) below plus 6.
There is a pressing need for a method in which sulfur and/or the sulfur content of an inorganic sub-six-sulfur-containing impurity in a water stream is oxidized to sulfate by molecular oxygen without employing a heavy metal sub-six sulfur oxidation catalyst. Such a catalyst, for example a copper salt, is, in general, at least as undesirable an environmental contaminant for a water stream as are the sub-six sulfur impurities herein.
2. Description of the Prior Art
A substantial effort has been directed to the improvement of water streams which contain inorganic sulfur-containing impurities in which the sulfur is in an oxidation state below +6. These impurities are a problem, because they impose an oxygen demand upon the aqueous environment. This demand may, and often is, preemptive relative to the oxygen need for one or more of the natural flora and fauna of the environment. Sulfur in a sub-six (below +6) oxidation state is especially undesirable in a waste water stream such as a sewage stream, and the like. Such streams must be treated with chlorine in order to eliminate harmful bacteria and the like which are present in the sewage. Inorganic sub-six-sulfur-containing impurities, however, interact preferentially with chlorine. Hence, the amount of chlorine required for a satisfactory sterilization of sewage is greatly increased when the sewage contains sulfur and/or sub-six-sulfur-containing compounds. These and other problems caused by inorganic sub-six-sulfur-containing compounds are treated in the reference book Aqueous Waters from Petroleum and Petrochemical Plants, by Milton R. Beychok, John Wiley & Sons (1967) . Minor amounts of fully oxidized sulfur --sulfate sulfur-- on the other hand have not appeared to be deleterious to the environment and are often a natural product.
In the article "New Column Removes Sulfide with Air," Hydrocarbon Processing and Petroleum Refiner, May 1962, Vol. 41, pp. 149-153, J. D. Martin and L. D. Levanas, a mechanical unit is described for use in a method for the purification of sulfide-bearing process waters by air oxidation. An earlier article, "Ridding Process Waters and Caustic Solutions of Sulfides," The Oil & Gas Journal, Vol. 54 (1956), pp. 95-99, relates to technical details of an oxidative conversion of sulfide. The product is mainly thiosulfate. The same or a similar result is described in the case of the Martin-Levanas article. Thiosulfate is an impurity relative to the environment, and is also undesirable as a contaminant in a waste water stream, though less so than sulfide because of its lesser oxygen demand upon the environment. Two articles by Otto Abegg in the publication Erdol und Kohle, Erdgas, Petrochemie: (1) Vol. 14, No. 8, pp. 621-626 (1961); and (2) Vol. 15, No. 9, pp. 721-722 (1962), describe the results of a study having as its object a complete conversion of sulfide to sulfate. Abegg, however, reports only being able in the absence of a catalyst to obtain at best a product stream sulfate-to-thiosulfate distribution of 75:25%, respectively. Abegg discloses in the second paper a more complete conversion of thiosulfate to sulfate by using a heavy metal catalyst --a copper sulfide catalyst-- but found that the copper catalyst was unsatisfactory because of corrosivity effects and of other problems. In terms of undesirable pollution effects, the impurities resulting from the use of a copper catalyst are probably more undesirable than the thiosulfate.
Canadian Patent No. 601,035 (7/5/60) relates to the oxidation of sulfidic-sulfur contents of waste water streams to thiosulfate.